The Effect of the Tides

The Sun and especially the Moon are the prime creators of our tides. The Moon has the most effect as it is the nearest body to the Earth. When the Sun and the Moon are in line or at opposites then the tides are the highest and these are called "spring tides"; this is when there is a New Moon or Full Moon. There is a lapse of one and a half days after full/new moon when Spring tides materialise. They occur every two weeks with the lowest tides called neaps intervening.

Tides have two effects,

1. Tidal movement Laterally
2. Tidal movement Vertically
   1. Lateral tidal movement or tidal streams (as it is called) usually move along the coast in first one direction building up to a maximum speed at around half tide and then in the opposite direction. With speeds varying from say 1knot to in extreme cases of 7knots depending on the topography of the coast. A knot is 1 sea mile per hour which is about 1.1 land miles. With relatively slow moving sailing boats it is advantageous to use the tidal 'lift' to significantly increase the boat speed and so shorten the journey time. Below are two chartlets of tidal streams and the rate /direction at two specific times relative to high water Dover plus 1 hour and high water Dover plus 6 hours.

2. Tides have approximately a twice daily cycle ie, two high waters and two low waters each day. In fact tide times, in keeping with lunar time, progress daily by on average half an hour.
   See below extract from Macmillan Reeds Nautical Almanac that shows clearly the differing tidal levels along with some names/explanations.